Aluminum alloy for cast node of vehicle space frames and method of manufacturing cast node using the same

ABSTRACT

The aluminum alloy of a vehicle space frame includes aluminum as a main ingredient, 0.4 to 0.5 wt % of copper, 2.5 to 3.0 wt % of silicon, 0.1 wt % or less of magnesium, 0.1 wt % or less of iron and 0.2 wt % or less titanium. The method of manufacturing the vehicle space frame includes casting the above material, subjecting the cast aluminum alloy to a solution treatment, and heat treating the resulting material at 150° C.-160° C. for 4-5 hours.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based on, and claims priority to Korean ApplicationNo. 2004-0035908, filed on May 20, 2004, the disclosure of which ishereby incorporated by reference.

BACKGROUND

1. Field

The present invention relates to an aluminum alloy for a cast node of avehicle space frame and a method of manufacturing the cast node usingthe same. More particularly, the present invention relates to how therange of adding each of the ingredients of the aluminum alloy isappropriately adjusted and an optimal heat-treating condition accordingto the above range of adjustment is presented, so that manufacturedproducts have a high elongation equivalent to that of productsmanufactured through a high-vacuum die-casting process, even through asand mold casting process is used.

2. Background of the Related Art

Aluminum has been expanding its uses while primarily being used in theautomobile industry where there is a need for improved fuel consumption.Particularly, the development of the design and manufacturing technologyof vehicle bodies using aluminum is progressing rapidly because of thelightness of aluminum.

Such developments are exemplified by an aluminum space frame (ASF),which is a vehicle structure used to reduce vehicle weight. Thecomponents constituting the space frame of a vehicle body are fabricatedfrom an extended aluminum material, and a cast connection member, i.e.,a cast node, is applied for joining the aluminum extrusion components.Then, an inner plate and an outer plate of the vehicle body are made ofan aluminum alloy plate material. Accordingly, the aluminum space frameis formed of a structure in which the extrusion material frame and thecast node absorb all the loads applied to the vehicle body.

Ensuring crashworthiness of vehicles is very important. Particularly, ina collision between vehicles, it is required that the vehicle body bedeformed at a certain portion while appropriately absorbing impactenergy to safely protect passengers in the vehicle. Therefore, thealuminum space frame itself must also have the characteristics in whichit is properly deformed without easy fracturing of the frame and thecast node while absorbing impact energy. That is, the material of theframe and the cast node must have high elongation to securecrashworthiness through the impact energy-absorbing capacity.

However, an aluminum casting product generally has low elongation; forexample, a sand mold casting product has an elongation of approximately3% and a metal mold casting product has an elongation of approximately 6to 7%. Therefore, it is not suitable for components of a vehicle body,such as the cast node, which require high elongation.

Accordingly, a thin-walled high elongation cast node with a thickness of2 to 3 mm is mass-fabricated using a newly developed alloy (Aural:Al-10%, Si—Mg, Mn, Fe) via a high-vacuum die-casting process and isapplied to an aluminum space frame (ASF) vehicle of some automobilemakers (Audi A2, A3 of Germany). The cast node made of such a materialhas physical properties of: tensile strength of 200 MPa, yield strengthof 130 Mpa and an elongation of 14 to 18%, and particularly has asufficient elongation suitable for a component of the vehicle body.

However, in case of using the high-vacuum die-casting process, since itsfacility itself is expensive and is based on a metal mold casting, theinvestment expenditure including the equipment cost, metal mold, etc.,is very costly. Especially where a small quantity of finished productsor test samples are fabricated, there occurs a problem in that anenormous burden of the cost and development cost is placed on abusiness.

Conversely, since molten metal is poured into a sand mold during castingto produce an object of a desired shape, the investment expenditure forthe product using sand mold casting is relatively low; thus beingsuitable for small batch production. The space frame body structure ismainly applied to a multi-product small batch production model.

However, an aluminum sand casting product has a relatively poor physicalproperty, for example, a low elongation of approximately 3% as comparedwith the metal casting product, which results in difficulty in itsapplication to manufacturing components of a vehicle body. There istherefore an urgent need for manufacturing a high elongation cast nodeby applying the sand casting process to the vehicle body components.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide analuminum alloy for a cast node of a vehicle space frame and a method ofmanufacturing the cast node using the same, in which the range foradding each of the ingredients of the aluminum alloy is appropriatelyadjusted and an optimal thermal treatment condition according to theadjustment of the addition range is presented, so that it enables tomanufacture products having a high elongation property equivalent tothat of products manufactured through a high-vacuum die-casting process,even through a sand mold casting process.

To accomplish the above object, according to one aspect of the presentinvention, there is provided an aluminum alloy for a cast node of avehicle space frame, which comprises aluminum as a main ingredient, 0.4to 0.5 wt % of copper, 2.5 to 3.0 wt % of silicon, 0.1 wt % or less ofmagnesium, 0.1 wt % or less by weight of iron and 0.20 wt % or less oftitanium.

According to another aspect of the present invention, there is alsoprovided a method of manufacturing a cast node of a vehicle space frame,which comprises the steps of: casting an aluminum alloy which comprisesaluminum as a main ingredient, comprises 0.4 to 0.5 wt % of copper, 2.5to 3.0 wt % of silicon, 0.1 wt % or less of magnesium, 0.1 wt % or lessof iron and 0.20 wt % or less of titanium; subjecting the cast aluminumalloy to a solution treatment; and heat-treating the resulting materialat a temperature of from 150 to 160° C. for 4 to 5 hours.

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features and advantages of the presentinvention will be apparent from the following detailed description ofthe preferred embodiments of the invention in conjunction with theaccompanying drawing, in which:

FIG. 1 is a graph of a thermal treatment curve showing the relationshipbetween time and temperature in accordance with the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENT

Reference will be now made in detail to the preferred embodiment of thepresent invention with reference to the attached drawing.

In order to fabricate a cast node used for joining between frameextrusion materials as components of an aluminum space frame, it isnecessary that a high elongation property be ensured for the case node.Accordingly, the present invention provides a sand casting alloy havinggood castability and a high elongation property to securecrashworthiness through the impact energy-absorbing capacity, and amanufacturing method of a cast node using the sand casting alloy,especially an optimal heat-treating condition to secure sufficientphysical property in the manufacturing process, so that the sand castingalloy can be fabricated for use as a complex vehicle body component.

Since the aluminum alloy proposed in the present invention has athin-walled and multi-rib structure in terms of the vehicle bodycomponent characteristics and is used for fabricating a large-sizedcomponent, it must have good castability and high elongation.

To this end, an aluminum alloy for a cast node of a vehicle space frameaccording to the present invention is characterized in that it is analuminum-copper-silicon alloy, which comprises aluminum as a mainingredient, 0.4 to 0.5 wt % of copper, 2.5 to 30 wt % of silicon, 0.1 wt% or less of magnesium, 0.1 wt % or less of iron and 0.20 wt % or lessof titanium.

At present, the alloy for use in sand casting or gravity casting ismainly AC4C (A35)-series alloy, which comprises about 6.5 to 7.5 wt % ofsilicon based on 100% by weight of the alloy (KS D 6008). The contentsfor respective ingredients of the high elongation sand casting aluminumalloy of the present invention and the AC4C aluminum alloy are given inthe following Table 1. TABLE 1 Composition and Content (wt %) Cu Mg SiFe Ti Al AC4C Less than 0.2-0.45 6.5-7.5 Less Less Residue 0.2 than than0.35 0.20 Aluminum 0.4-0.5 Less than 2.5-3.0 Less Less Residue Alloy 0.1than than 0.1 0.20

In the composition and content range of the present invention given inthe above Table 1, copper has a solid solution strengthening effect anda precipitation strengthening effect. However, if the solid solution andprecipitation strengthening effects are very strong, there is a risk ofdegrading corrosion resistance. For this reason, the content of copperwas limited to a range of 0.4 to 0.5 wt %. In addition, if silicon isadded to the aluminum alloy, fluidity is improved. That is, silicon isan element that is added in a large amount to increase castability.However, the content of silicon was limited to a range of 2.5 to 3.0 wt% in the present invention.

Also, magnesium has the same precipitation strengthening effect as thatof silicon, but if excessive precipitation occurs during the thermalaging process, this adversely affects elongation of the aluminum alloy.Accordingly, the content of the magnesium (Mg) was limited to a range ofless than 0.1 wt %. Moreover, since iron forms a compound together withaluminum and silicon to thereby degrade elongation of the aluminumalloy, the content of iron was limited to a range of less than 0.1 wt %.

In the meantime, the inventive method of manufacturing a case node of avehicle space frame using the aluminum alloy having the abovecomposition, is characterized in that after casting the aluminum alloy,the cast aluminum alloy is subjected to a solution treatment, followedby artificial thermal aging at a temperature of from 150 to 160° C. for4 to 5 hours.

FIG. 2 is a graph of a thermal treatment curve showing the relationshipbetween time and temperature in the manufacturing method of the casenode in accordance with the present invention. Referring to FIG. 2, apreferred embodiment of the present invention is described, in whichafter the aluminum alloy has been cast, it experiences a solutiontreatment at a temperature of 538° C. for 8 hours, followed byartificial thermal aging at a temperature of 155° C. for 4 hours.

In the manufacturing method, if the thermal aging temperature is set toa temperature range of less than about 150° C., an appropriate agingdoes not occur. Accordingly, a necessary physical strength is notachieved and thus this condition is not desirable. If, on the otherhand, the thermal aging temperature is set to a temperature range ofabove about 160° C., elongation property is deteriorated due to anexcessive aging. Consequently, this condition is also not desirable.

Further, if the thermal aging time is set to less than about 4 hours atthe above-mentioned temperature range, i.e., at a temperature of fromabout 150 to 160° C., an appropriate aging does not occur. Accordingly,since a necessary physical strength is not achieved, this condition isnot desirable. And if the thermal aging is conducted for more than about5 hours, an elongation property is deteriorated due to excessive aging.Consequently, this condition is also not desirable.

In this way, a cast node is cast using the aluminum alloy with the abovecomposition and content through a sand mold casting process, andundergoes a solution treatment at a predetermined temperature for acertain time period, followed by artificial thermal aging under thethermal treating condition proposed by the present invention, so that asufficient physical property required by a cast node of a vehicle spaceframe can be ensured.

The preferred embodiment of the present invention will now be describedin more detail hereinafter.

EXAMPLES AND COMPARATIVE EXAMPLES

In this embodiment, after the aluminum alloy, after casting, wassubjected to a solution treatment at a temperature of from 538° C. for 8hours, followed by thermal aging at a temperature of 155° C. for 4 hoursby using the aluminum alloy with the composition and content suggestedby the present invention through a sand mold casting process.

Tensile strength, yield strength an elongation were measured using asample manufactured through the above method. The result of themeasurement was given in the following Table 2 along with the physicalproperties of AC4C alloy widely used in sand mold casting. TABLE 2Thermal Tensile Yield treatment strength strength condition (Mpa) (Mpa)Elongation (%) AC4C After solution 230 190 2.8 treatment, thermal agingat a temperature of 160° C. for 6 hours Example 1 After solution 242 13218.3 treatment, thermal aging at a temperature of 155° C. for 4 hours

It is shown from the Table 2 that the yield strength of the aluminumalloy (Example 1) is relatively low as compared with that of aconventional alloy material (AC4C), but the inventive aluminum alloyexhibits a relatively high elongation of 18.3% that cannot be achievedby a general cast product and can ensure a similar physical property tothat obtained by a high-vacuum die-casting process. For example, thealloy product applied to Audi A2, A3 vehicles of Germany exhibits atensile strength of 200 MPa, an yield strength of 130 MPa and anelongation of from 14 to 18%.

In the meantime, the aluminum alloy having the same composition andcontent as suggested by the present invention was subjected to a sandcasting and a solution treatment under the same condition as inmanufacturing the inventive cast product, followed by a thermal agingprocess under different temperature and time conditions to obtainfabricate respective samples. The evaluation results for the physicalproperties of the respective samples are given in the following Tables 3and 4.

Table 3 shows the result of comparison between the physical propertiesof the respective samples according to a variation in thermal agingtemperature, and Table 4 shows the result of comparison between thephysical properties of the respective samples according to a variationin thermal aging time. TABLE 3 Tensile Yield Thermal aging strengthstrength temperature (° C.) (MPa) (MPa) Elongation (%) Example 1 155 242132 18.3 Comparative 145 210 121 18.7 Example 1 Comparative 160 249 14316.2 Example 2 Comparative 170 260 154 12.3 Example 3

TABLE 4 Tensile Yield Thermal aging strength strength time (hr) (MPa)(MPa) Elongation (%) Example 1 4 242 132 18.3 Example 2 5 248 140 17.8Comparative 3 185 121 16.1 Example 4 Comparative 6 252 141 12.1 Example5 Comparative 7 262 158 8.2 Example 6

The thermal aging time in Example 1 and Comparative Examples 1 to 3 ofthe above Table 3 was set to be identical to one another i.e., 4 hours,and the thermal aging temperature in Examples 1 and 2 and ComparativeExamples 4 to 6 of the above Table 4 was set to be identical to oneanother, i.e., 155° C.

The measurement result for the physical properties of the respectivesamples shows that Examples 1 and 2 can obtain a desired physicalproperty suitable for a cast node of a vehicle space frame, for example,a high elongation. It can be seen from the result that the thermaltreatment condition depending on the temperature and time suggested bythe present invention is optimal.

As described above, according to an aluminum alloy for a cast node of avehicle space frame and a method of manufacturing the cast node usingthe same, the range for adding each of the ingredients (copper, silicon,magnesium and iron) of the aluminum alloy is appropriately adjusted andan optimum heat-treatment condition according to the adjustment of theaddition range is presented, so that it enables manufactured products tohave the physical properties, such as tensile strength, yield strengthand elongation, equivalent to those of products manufactured through ahigh-vacuum die-casting process, even through a sand mold castingprocess is used.

That is, although the high-vacuum die-casting process is not applied tothe manufacturing process of aluminum vehicle body components, itenables one to fabricate the aluminum vehicle body components having thephysical properties equivalent to those of products manufactured throughthe expensive and complex high-vacuum die-casting process, by using alow-priced sand mold casting process, thereby greatly reducing theproduct development expenditure, cost and complexity.

While the present invention has been described with reference to theparticular illustrative embodiments, it is not to be restricted by theembodiments but only by the appended claims. It is to be appreciatedthat those skilled in the art can change or modify the embodimentswithout departing from the scope and spirit of the present invention.

1. An aluminum alloy for a cast node of a vehicle space frame, whichcomprises aluminum as a main ingredient, 0.4 to 0.5 wt % of copper, 2.5to 3.0 wt % of silicon, 0.1 wt % or less of magnesium, 0.1 wt % or lessof iron and 0.2 wt % or less of titanium.
 2. A method of manufacturing acast node of a vehicle space frame, which comprises: casting an aluminumalloy which comprises aluminum as a main ingredient, 0.4 to 0.5 wt % ofcopper, 2.5 to 3.0 wt % of silicon, 0.1 wt % or less of magnesium, 0.1wt % or less of iron and 0.20 wt % or less of titanium; subjecting thecast aluminum alloy to a solution treatment; and heat-treating theresulting material at a temperature of from 150 to 160° C. for 4 to 5hours.
 3. An aluminum alloy for a vehicle space frame, said aluminumalloy comprising aluminum; about 0.4 to 0.5 wt % copper; about 2.5 to3.0 wt % silicon; at most about 0.1 wt % magnesium; at most about 0.1 wt% of iron; and t most about 0.2 wt % titanium.
 4. A method ofmanufacturing a vehicle space frame, which comprises: casting analuminum alloy which comprises aluminum; about 0.4 to 0.5 wt % copper;about 2.5 to 3.0 wt % silicon; at most about 0.1 wt % magnesium; at mostabout 0.1 wt % of iron; and t most about 0.2 wt % titanium; subjectingthe cast aluminum alloy to a solution treatment; and heat-treating theresulting material at a temperature of from about 150 to about 160° C.for about 4 to about 5 hours.